Fig 1Speed from framesF fps · S speed · D span · space replay18.5 m ÷ (17 ÷ 30 fps) = 117.5 km/h
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Between the cursors on the tape: the distance you divide byThe ball once per video frame; the lit ones are release and the arrival frameTiming strip: one tick per frame, the lit inlay is ±1 frame at arrivalWhat the phone must see: the release and the batter's stumpsThe stumps are 20.12 m apart (Law 6), but the ball leaves the hand about 1.6 m in front of the bowler's stumps, so it travels about 18.5 m to the batter's stumps. Speed = distance ÷ (frames ÷ fps). Neither release nor arrival falls exactly on a frame, so the count can be one frame either way: at 30 fps a 120 km/h ball moves 1.11 m a frame and reads 117.5 km/h (111.0–124.9), about ±6%; at 240 fps it moves 0.14 m and the band is under ±1%. Dividing by 20.12 m instead of 18.5 m reads about 9% fast. The ball is drawn at a constant speed, the average a frame count measures; release height, the ball's line and the camera's distance are illustrative, and the ball and phone are drawn larger than life.
Stance Cricket™Stance Lab · Speed